This is somewhat dated but it may help to select a theme other than oxygen+gtk.
http://blog.kokuaviewer.org/2013/02/18/kokua-3-4-5-25935-is-available/comment-page-1/#comment-5127
Also, Kokua recently corrected an issue with web-ket plugin fail and a Nightly build viewer is avaiable with that corrected.
http://sourceforge.net/projects/kokua.team-purple.p/files/Nightly/kokua-x86_64-3.6.11.30665.tar.bz2/download